In the course of attempting to recover data transmitted over a communication channel, a digital receiver must set the input level of the aggregate desired signal plus noise and interference, henceforth referred to as the aggregate signal, at the input to the analog-to-digital converter such that effects of quantization on the desired signal are minimized. The aggregate signal level in to the analog-to-digital converter is set using a variable gain stage somewhere in the analog front-end preceding the digital receiver. If the gain of the analog front end is set to low, quantization noise will dominate and the number of effective bits of the analog-to-digital converter will be reduced. If the gain is set to high, the analog-to-digital converter will saturate, distorting the waveform and preventing the successful recovery of the data. Variation in the communication channel's propagation properties and the presence or absence of interference necessitate the use of some sort of automatic gain control to keep the aggregate signal level into the analog-to-digital converter at an optimal value over a large dynamic range.
To complicate matters, communications systems subjected to one or more sources of interference, intentional or otherwise, often employ some form of frequency hopping scheme, through which the receiver transitions or ‘hops’ from (the center frequency of) one frequency bin to another, and looks for energy in the hopped-to bin, during a relative brief ‘hop’ interval. One of the problems that must be faced with this type of system is the fact that the receiver doesn't know a priori which frequency bin will possess interference.
For example, there may exist a relatively narrowband jammer emitting in or near a frequency bin to which the receiver hops only occasionally. If the gain level of the input to the receiver's analog-to-digital converter (ADC) is set at some mean value, then hopping to the jamming band will result in saturating the ADC, causing a data packet to be occasionally lost, thereby degrading system throughput. On the other hand, if the gain is set at a value based upon the impact of the jammer in that one band, then when the receiver is hopping to and collecting energy in other bands, the ADC is underloaded, introducing quantization noise and causing an unwanted degradation in performance.
Moreover, the use of frequency hopping over wide bandwidths causes additional aggregate signal level dynamics, as a function of frequency, due to the large gain variations typical of cost-effective analog frontends.
The prior art has customarily set the gain at a value based on the mean value of the aggregate input signal level over the entire spread bandwidth (with some overhead reserved for jamming) and then attempts to rely on interleaving and decoding to overcome the performance degradation caused by not optimally loading the ADC.